1. Field of the Invention
The present invention relates to toners for developing electrostatic images of electrophotography, electrostatic recording, electrostatic printing, and the like; and to developers, toner containers, process cartridges, image forming apparatuses, and image forming methods using the toners.
2. Description of the Related Art
Image formation by e.g. electrophotographic method is generally carried out by a series of processes including: forming a latent electrostatic image on a photoconductor (latent electrostatic image bearing member); developing the latent electrostatic image by a developer containing a toner to form a visible image (toner image); then transferring the visible image to a recording medium such as paper; and fixing the image to form an fixed image.
The toner is a colored particle comprising a binder (binder resin), colorant, charge controlling agent, etc. which are contained in the binder. As the method for producing the toner, pulverization and suspension polymerization are mainly known.
The pulverization is a method for producing a toner in which a colorant, charge controlling agent, etc. are melt-mixed and are uniformly dispersed into a binder to obtain a toner composition, and the obtained toner composition is grinded, classified, etc. to form a toner. The pulverization has drawbacks as follows. Specifically, a grinder, etc., are required to grind a toner composition, resulting in high cost, and thus the method is not effective. In addition, during the grinding, toner particles with wide distribution of particle diameter tend to be formed. Therefore, in order to obtain images with high resolution and high gradation, a portion of the toner particles, for example, minute particles of 5 μm or less in diameter and large grains of 20 μm or more, must be removed by classification, inviting a significant reduction of yield. Furthermore, it is difficult to disperse additives such as a colorant, and charge controlling agent into the binder uniformly. The use of the toner in which the additives are not dispersed uniformly deteriorates flowability, developability, durability, image quality, etc.
Recently, to overcome these problems in pulverization, a method for producing a toner by polymerization of monomer is proposed and carried out. For example, toner particles are produced by suspension polymerization. However, toner particles obtained by suspension polymerization are generally spherical and have drawback of poor cleaning ability poor cleaning ability causes non-transferred residual toner on a photoconductor, and the accumulation of such residual toner leads to background smear. Moreover, residual toner contaminates components such as a charging roller, which charges a photoconductor by contact charging, and subsequently reduces the charging performance of the charging roller.
Therefore, a method for producing toner particles is proposed in which emulsion polymerization is used to form resin fine particles, which are subsequently associated to obtain toner particles having irregular shapes (See Japanese Patent (JP-B) No. 2537503). However, toner particles formed by emulsion polymerization have residual surfactants in large amounts inside the particles as well as on the surface thereof even after being washed by water. As a result, charge stability of toner is reduced, the distribution of the amount of charge is increased, causing background smear on a printed image. In addition, the residual surfactant contaminates photoconductor, charging roller, developing roller, etc. Therefore, toner cannot fulfill its original function.
On the other hand, for the fixing process by contact heating, in which heating members such as a heating roller are used, the toner particles must possess releasability, which may be referred to as “offset resistance” hereinafter, from the heating members. In such case, offset resistance can be improved by allowing a releasing agent to exist on the surface of toner particles. In contrast, methods to improve offset resistance are disclosed in which resin fine particles are not only contained in toner particles, but are concentrated at the surface of the toner particles (See Japanese Patent Application Laid-Open (JP-A) No. 2000-292973 and JP-A No. 2000-292978).
These proposals, however, cause increase of lowest fixing temperature, resulting in unsatisfactory fixing ability at low temperatures, i.e. energy-saving fixing ability. In addition, this method, in which resin fine particles obtained by emulsion polymerization are associated to provide irregular-shaped toner particles, has another problem. Generally, releasing agent particles are additionally associated to improve the offset resistance. However, the releasing agent particles are captured inside the toner particles and therefore the improvement of the offset resistance is not sufficient. Moreover, since each toner particle is formed by a random adhesion of molten resin fine particles, releasing agent particles, colorant particles, and the like, the composition (the ratio at which each component is contained), molecular mass of the resin, and the like may be different and dispersed for each obtained toner particle. In result, the surface properties of toner particles are different from one another, and it is impossible to form stable images for a long period. Additionally, in a low-temperature fixing system, the resin fine particles that are concentrated at the surface of the toner particles inhibit fixing and therefore the range of fixing temperature is not sufficient.
Recently, a new method for producing a toner, called solution suspension method (Emulsion-aggregation method (EA method), has been suggested (See JP-B No. 3141783). In this method, particles are formed from polymers that are dissolved in an organic solvent or the like whereas in suspension polymerization, polymer particles are formed from monomers, and the method is advantageous in that, for example, there is a larger selection of resins that can be used and polarity can be controlled. Furthermore, the method is advantageous in that it is possible to control the structure of toner particles (core/shell structure control). However, the shell structure is a layer consisting only of a resin and the purpose thereof is to lower the exposure of pigment and wax to the surface. The purpose is not to alter the structure in the resin, and the structure is not capable for such purpose (See “The characteristics of newly developed toner and the vision for the future” by Takao Ishiyama, and two others from The 4th Joint Symposium of The Imaging Society of Japan and The Institute of Electrostatics Japan on Jul. 29, 2000). Therefore, although the toner particle has a shell structure, the surface of the toner particle is a usual resin without any ingenious feature so that when the toner particle is targeted at fixing at a lower temperature, there is a problem that it is not satisfactory from the standpoint of anti-heat preservability and environmental charge stability.
In any of the conventional methods such as the suspension polymerization, emulsion polymerization, and solution suspension, styrene-acrylic acid ester copolymer is used as a binder resin in many cases. Polyester resins are not generally used because they are difficult to be made into particles, it is uneasy to control particle diameter, diameter distribution, particle shape, etc., and their fixing ability is insufficient under the condition of fixing at a lower temperature.
In pulverization, in order to achieve fixing at low temperatures, a polyester resin having a high acid value is used. For example, JP-B No. 3141783 and JP-A No. 09-204071 propose toners comprising a resin of which acid value, hydroxyl value, molecular mass distribution, THF insoluble content, or the like, are defined. The toner in these proposals, however, causes the reduction of melting temperature at the same time, resulting in the deterioration of offset resistance. In order to achieve all of fixing property at low temperatures, offset resistance and anti-heat preservability, further improvement is needed.
Much work has been done from various angles of approach in the field of electrophotography to improve quality, and it is being recognized that it is extremely effective to reduce the size and increase the sphericity of the toner particle. However, as the diameter of toner particles becomes smaller, the transferability and fixing ability tend to decrease, and image quality becomes poor. Especially, with respect to fixing, fixing ability at a halftone portion becomes worse. This is because at the halftone portion, the adhesive amount of toner is low, the toner, transferred to the concave portion on a transfer material, is given extremely small amount of heat from a fixing roller, causing generation of offset phenomenon easily. In addition, it is known that by making toner particles round, the transferability rises (See JP-A No. 09-258474).
In such situation, ever-faster image production is desired in the field of color copiers and printers. For a faster printing, the “tandem method” is effective (See JP-A No. 05-341617). The “tandem method” is a method in which images formed by respective image forming units are overlaid and sequentially transferred onto a sheet of paper that is advanced by a transfer belt so that a full-color image is obtained on the sheet. In a color image forming apparatus using tandem method, various kinds of paper can be used, the quality of full-color images is high, and full-color images can be formed at high speed. The high-speed output of full-color images is especially characteristic and no other color image reproduction machines have that characteristic. There are other attempts to increase speed while improving the quality by using round toner particles. In order to increase speed more, the round toner is required to be fixed quickly; however, in a present situation, such round toner that has both quick fixing ability and fixing ability at low temperature has not been achieved.
Toner may be subjected to severe circumstances such as high temperature and humidity, and low temperature and humidity during storage and transport after the production. There has been a demand for a toner which does not aggregate to each other, of which flowability, transferability, and fixing ability do not deteriorate or rarely do, and which has excellent preservability, even after storage for a long period under such circumstances. However, in the present situation, effective means for such demand has not been found especially with respect to spherical toner.
In electrophotographic system, a heat-pressure fixing method by means of a heating roller is conventionally used. In the method, while the surface of a heat roller possessing releasability for a toner is brought contact with the toner image on the surface of a receiving sheet under pressure, the receiving sheet is allowed to pass through to thereby fix the toner image. In this method, the surface of the heat roller and toner image on the receiving sheet are brought into contact with each other under pressure. Thus, heat efficiency during the melt-fixing of toner image on the receiving sheet is extremely satisfactory, which enables quick fixation.
By the way, in the heat-pressure fixing method by means of a heating roller, the surface of the heating roller and toner image are brought into contact with each other in a melted state and under pressure. A portion of toner image is transferred to the surface of fixing roller to adhere, and the transferred portion of toner image is re-transferred to the next receiving sheet, leading to the pollution of the receiving sheet. This so-called offset phenomenon is greatly influenced by the fixing speed and fixing temperature. This is because almost constant amount of heat for fixing toner is given to toner without depending on the fixing speed.
In general, when the fixing speed is slow, the surface temperature of heating roller is set to relatively low temperature. In contrast, when the fixing speed is fast, the surface temperature of heating roller is set to relatively high temperature.
The toner on the receiving sheet forms several toner layers. Thus, particularly, in a system where fixing speed is fast, the surface temperature of heating roller is high, an uppermost layer of toner layers which contacts with a heating roller and a lowermost of toner layers which contacts with the receiving sheet temperature difference becomes large. Therefore, when the surface temperature of heating roller is high, the toner of the uppermost layer tends to cause offset phenomenon, and when the surface temperature of heating roller is low, toner does not fix to the receiving sheet because the toner of the lowermost layer does not melt sufficiently, causing low-temperature offset phenomenon easily.
As a way to solve this problem, when the fixing speed is fast, a method is normally carried out in which pressure during fixing is increased, making the toner to be anchored to the receiving sheet. This method can reduce heating roller temperature to some degree, and can prevent high-temperature offset phenomenon of the uppermost layer of toner layers. However, shearing force on the toner becomes very large, receiving sheet winds around the fixing roller, i.e., so-called winding offset occurs, and a trace of separating pawls for separating the receiving sheet from the fixing roller is likely to appear on a fixed image. Further, inferior fixed images are likely to occur, such as such as failure of line images during fixing and toner scattering, due to a high pressure.
Moreover, in a high-speed fixing system, a toner having a lower melt viscosity is generally used than in the case of low speed fixation, and the surface temperature of the heating roller and fixing pressure are lowered. Thus, a toner image is fixed while obviating the high-temperature offset and winding offset. However, the use of such a toner having a low melt viscosity in low speed fixation is likely to cause an offset phenomenon at high temperature.
Accordingly, in fixing, there has been demand for a toner which shows a wide fixable temperature range and excellent offset resistance and is applicable from a low-speed apparatus to a high-speed apparatus.
In order to obtain high quality image, an attempt to make the size of toner particles smaller has been made. Smaller particle size toner increases the resolution and clearness of an image, but impairs the fixability of a halftone image. This phenomenon is particularly noticeable in high-speed fixation. This is because the adhesive amount of toner in a halftone part is small and the toner transferred to a concave portion of a receiving sheet receives only a small quantity of heat from a heating roller and the pressure applied thereto is also suppressed because of the convex portion of the receiving sheet. The toner transferred onto the convex portion of the receiving sheet in a halftone part receives a larger shearing force per toner particle because of thin toner layer thickness, compared with that in a solid image part with thick toner layer thickness. Thus, offset phenomenon is likely to be caused and fixed image is likely to have low quality.
Until now, in order to pursue fixing performance and anti-hot offset, a variety of studies, mainly on binder resin, have been made. For example, JP-A No. 05-107803 proposes resin having a molecular mass distribution such that the distribution has at least one local maximal value in each of the region of a molecular mass of 103 to 7×104 and the region of a molecular mass of 105 to 2×106 in a chromatograph by gel permeation chromatography (GPC) of resin for toner. Further, in JP-A No. 05-289399 and JP-A No. 05-313413, the molecular mass distribution of vinyl copolymer is defined and releasing agent such as polyethylene is added to pursue fixing ability and hot offset. Furthermore, in JP-A No. 05-297630, by combining a resin having low viscosity with resin having high viscosity, an attempt to improve fixing property at low temperatures and hot offset property simultaneously is made. In other Patent Literatures, many techniques have been proposed that pursue optimization of balance of preservability, fixing ability, and hot offset that are difficult to pursue simultaneously by widening the molecular mass distribution of binder resin (See e.g. JP-A No. 05-289399, 05-313413, 05-053372, 06-027733, 06-075426, and 06-118702).
In electrophotography, anti-heat preservability, which is influenced by elements with a low molecular mass, must be also satisfied besides these two properties that are difficult to pursue simultaneously. For example, in JP-A No. 08-146661, attempts to improve anti-heat preservability, etc. by using a novolac type phenol resin or polyurethane other than molecular mass distribution have been made.
In these proposals, the effect by defining the molecular mass distribution or the effect by olefin having low molecular mass improves fixing at low temperatures and anti-heat preservability; however, these binder resins do not meet the recent demand for energy-saving and low-power enough and further investigation has been desired.
In particular, in order to improve fixing property at low temperatures, it is required to lower the glass-transition temperature (Tg) and molecular mass of binder resin. However, in the present situation, it is difficult to develop such toner that satisfies all these properties in light of balance between hot offset property and preservability.
For example, JP-A No. 11-133665 proposes a dry toner containing a urethane-modified polyester (A) as a toner binder obtained by elongation reaction and having a practical sphericity of 0.90 to 1.00 in order to improve the fluidity, fixing property at low temperatures, and hot offset property. Further, a dry toner is proposed that has excellent powder fluidity and transferability, although the toner has a small particle diameter, and is also excellent in any of anti-heat preservability, fixing property at low temperatures, and hot offset resistance. The dry toner produces glossy images, especially, when used in e.g. a full-color copier and does not require application of oil to a heat roller.
Although the dry toner proposed by JP-A No. 11-133665 is novel in that binder obtained as a result of a urethane reaction is employed, it is produced by a pulverization process and does not have satisfactory fixing ability at low temperatures. In addition, specific conditions enabling a small particle diameter and controlling particle shape so as to be spherical are not described.
Moreover, JP-A No. 11-149180 and JP-A No. 2000-292981 propose a dry toner comprising a toner binder formed from an elongation and/or a crosslinking reaction of an isocyanate group-containing prepolymer, and a colorant, wherein the dry toner is formed of particles formed from an elongation and/or a crosslinking reaction of the modified polyester (A) by amines (B) in an aqueous medium. JP-A Nos. 11-149180 and 2000-292981 also propose a method for producing the toner, which is an economically affordable method to obtain a dry toner.
The toners proposed in these JP-A Nos. 11-149180 and 2000-292981 are prepared by granulation in water. However, in such granulation in water, a pigment in an oil phase aggregates at the interface with an aqueous phase, which leads to decreased volume resistivity or uneven pigment distribution and causes problems in fundamental properties of the toner. To achieve simultaneously a small particle diameter and a satisfactorily controlled shape of a toner for use in a machine without application of oil, the specific shape and/or properties must be defined and without such specified shape and/or properties, effect cannot be achieved. However, each Patent Literature dose not describe adequately the effects of the combination of properties and/or processes or effects of the balance between detailed conditions, and thus effects on the problems may not be significantly achieved. Particularly, in the case of toner particles prepared by granulation in water, pigment and/or wax is likely to gather on the surface of the particles of toner. Toner particles having a particle diameter of about 6 μm or less have a large specific surface area, thus design of the particle surface becomes important for achieving desired charging properties fixing properties in addition to the design of the polymer component.
In general conventional electrophotographic image forming apparatus comprises a heat fixing unit in which a pressure member such as a pressure roller is brought into contact with a heating member such as heating roller having a heat source inside thereof a recording medium on which image has been transferred is passed therebetween and while the recording medium being transported, toner images on the recording medium are fixed.
In this type of heat fixing unit, so-called offset phenomenon that toner on the recording medium adheres to a heating member may occur. It is known that when this offset phenomenon occurs, offset toner also adheres to the pressure member, and toner adhered to those heating member and pressure member is transferred back to a recording medium to contaminate the recording medium. In order to prevent the occurrence of offset, in a conventional heat fixing unit, for example, the surface of a heating member was coated with fluorine. However, it is difficult to prevent offset phenomenon completely depending on environmental conditions, types of recording medium, etc, eventually causing reverse transfer.
Therefore, a heat fixing unit is proposed in which a cleaning member such as cleaning roller is provided in contact with a heating member and pressure member to thereby remove toner adhered to the heating member and pressure member. In this heat fixing unit, cleaning member, made of pure metal material is brought into contact with a heating member or pressure member having improved surface releasability, thereby removing toner due to the difference of surface releasability.
Recently, an image forming apparatus has been constructed in the following manner in order to prevent a waste of energy. Specifically, during the stand-by state, current to the heat source of a heat fixing unit is stopped, only when image forming starts, current is allowed to flow to the heat source, and the temperature of the heating member is raised to the fixing temperature. Therefore, the heating member is required to have improved response to temperature, for example, a heating roller has a thickness of 1 mm or less, thereby shortening the time to rise to a fixing temperature to approximately 10 seconds.
In such image forming apparatus, the heating member of a heat fixing unit has a low thermal capacity, thus heat easily moves to a recording medium at the time of fixing or to a member contacting with the heating member, or the heating member is liable to be influenced by the flow of the wind around the heating member. These cause a problem that the temperature distribution of the heating member is likely to become uneven in the direction of width. Therefore, it is impossible to make the temperature distribution even over the entire region in terms of space and cost.
In a heat fixing unit, uneven temperature distribution of heating member in the direction of width leads to unstability of fixing performance, and at the same time, offset is likely to occur. In addition, there is a problem that deterioration by heat makes the lifetime of a heating member shorter. In particular, the use of polymerized toner produced by polymerization described in JP-A Nos. 11-133665 and 2000-292981 causes a problem that tone adheres to a cleaning member and accumulates thereon, and the masses of toner melt again and the toner is transferred back to a recording medium. This is because when pulverized toner produced by pulverization is used, the toner adhered to the cleaning member has a high storage modulus and is unlikely to melt; however, when polymerized toner produced by polymerization is used, the toner adhered to the cleaning member has a low storage modulus, as is expected to toner produced by polymerization.
This problem is caused especially when recording medium, e.g. a paper, with small size compared with maximum size to which sheet is run through is passed through. The reason for this is considered as follows. The passed region by a recording medium with small size is narrow and thus the contact area with a heating member is small. Therefore, only the narrow region has decreased temperature and temperature sensor corresponding to the region dictates switch-on of a heat source, resulting in unnecessary rise in temperature of the region where sheet has not passed. This causes the toner on a cleaning member corresponding to the region where sheet has not passed to melt and be transferred back.
In attempting to solve such problem of back transfer, JP-A No. 09-325550 proposes a heat fixing unit in which in order to make the temperature distribution of heating roller uniform in the direction of width, wind is applied, thereby preventing the region where sheet has not passed of the heating roller from having excessively raised temperature.
In addition, JP-A No. 2002-123119 proposes a heat fixing unit in which air holes are provided along a cleaning roller so that air in the heat fixing unit is circulated with rotation of the cleaning roller to thereby prevent the temperature of the cleaning roller from being raised.
However, there has not been provided a toner which can fix satisfactorily immediately after power activation and even under low-power condition, which has releasability applicable to from low-speed through high-speed image forming apparatuses, which is excellent in offset resistance, blocking resistance, and flowability, which does not affect fixing efficiency in a heat fixing unit, and which is not transferred back when adhered to a cleaning member; and related techniques. Thus, in the present situation, it has been desired that such toner and related techniques are provided as soon as possible.